The present invention broadly relates to systems used to transfer wafers in a semiconductor manufacturing process, and deals more particularly with a method and apparatus for monitoring and controlling the orientation of the wafers during a wafer transfer sequence.
The need for increasing yield and throughput in semiconductor manufacturing systems has resulted in the development of highly specialized and automated systems for processing and handling semiconductor wafers. Wafers are typically stored in a cassette with their flat surfaces horizontal. The cassettes are transferred from station-to-station by automatic material handling equipment. Once delivered to at a process station, individual wafers are transferred from the cassette by an automatically controlled robot that typically includes a robotic arm carrying a wafer supporting blade on its outer end. The blade is moved in a manner to pick up individual wafers such that the wafer lays flat on the blade during the transfer process. This arrangement has the advantage that a wafer is held on the end of the robotic arm through the force of gravity, thus avoiding the application of unnecessary or unbalanced forces to the wafer that could result in damage to the wafer.
Wafer processing systems of the type referred to above typically include one or more so called xe2x80x9ccluster toolsxe2x80x9d consisting of a modular, multi-chambered, integrated processing system having a central wafer handling module, and a number of peripheral process chambers. Cluster tools have become generally accepted as effective and efficient equipment for manufacturing advanced microelectronic devices. Wafers are introduced into a cluster tool where they undergo a series of processing steps sequentially in various circumferentially arranged chambers wherein wafer transfer is effected through the robotic wafer handling module which is located in the central region of the system. The wafer handling module includes a robotic wafer transfer arm which is positioned on a turntable that rotates through 360 degrees, thus permitting the arm to transfer wafers sequentially from chamber-to-chamber.
In spite of the fact that the robotic arm for transferring the wafers is automatically controlled, slight misadjustments, errors caused by wear or software xe2x80x9cglitchesxe2x80x9d can result in problems in the wafer transfer process. For example, in some cases, misalignment of certain components in the system, such as the blade, robotic arm or the cassette can result in a wafer being loaded onto a blade such that the wafer is tilted relative to the blade, i.e. not horizontal on the blade. In other cases, although the wafer may be supported flat on the blade, the blade itself may be tilted before it lifts a wafer from the cassette. In still other cases where the rotational position of the robotic arm relative to a cassette is not precisely indexed or where the blade is tilted out of its normal horizontal position, attempts to fetch a wafer from the cassette may result in the robotic arm or the blade colliding with slit door insert of load lock chambers which in turn damages or breaks the wafer. Even where the robotic succeeds in lifting a wafer from the cassette, alignment errors in the transfer process can result in collisions between process chamber pedestal, slit door, lower parts of chamber and the wafer during loading, unloading or transfer of the wafers.
The problem described above involving wafer damage due to errors in the transfer process have become more significant as the size of the wafers continues to increase, along with advances in wafer processing technology. This type of wafer damage reduces throughput and process yield, and is becoming more costly due to the larger size of the wafers.
The present invention is directed toward overcoming the deficiencies of the prior art wafer transfer systems described above.
According to one aspect of the invention, apparatus is provided for controlling the operation of a semiconductor wafer transfer robot of the type including a moveable robotic arm, a wafer blade for holding the wafer, a means for driving the arm. The apparatus includes means for sensing proper positioning of the wafer on the blade and control means for controlling the motor means which is responsive to the sensing means for stopping movement of the arm when a wafer is not properly positioned on the blade. The sensing means preferably includes a pair of lasers positioned to produce a corresponding pair of laser beams that pass across and are closely spaced from the surface of the wafer, along with a pair of laser beam sensors for respectively sensing the beams, wherein at least one of the laser beams is blocked when a wafer is not properly positioned on the blade. The control means includes logic means that allow movement of the robotic arm only when both laser beam sensors indicate that the wafer is properly positioned on them blade. The sensing means also includes means for sensing the position of the robotic arm to ensure that it is properly positioned to load and unload the wafers.
According to another aspect of the invention, apparatus is provided for transferring a semiconductor wafer between a plurality of chambers in a semiconductor processing system, comprising robotic arm means shiftable between a retracted home position and extended wafer transfer position; a blade disposed on the outer end of the arm upon which one of the wafers may be placed; means for rotating the robotic arm means between a plurality of indexed positions respectively corresponding to the locations of the chambers; drive means for driving the arm means to rotate between the indexed positions and to shift between the retracted and extended positions thereof; means for sensing when the wafer is properly positioned on the blade and when the arm means is properly positioned at one of the indexed positions thereof; and control means coupled with the sensing means and the drive means for controlling the operation of the drive means.
According to a further aspect of the invention, a method is provided for controlling the operation of a semiconductor wafer transfer system of the type having a blade on which a wafer is carried, a robotic arm for transferring the blade between a plurality of chambers in a semiconductor processing system, which includes the steps of: rotating the arm between each of a plurality of indexed positions, each corresponding to one of the chambers; shifting the arm between an extended, wafer transferring position, and a retracted, home position; sensing when the semiconductor is not in a pre-selected, desired position on the blade; and, inhibiting rotation and shifting of the arm when the wafer is not in the pre-selected position thereof.
Accordingly, it is the primary object of the present invention to provide a method and apparatus for detecting proper orientation of a semiconductor wafer and a wafer transfer system which reduces damage to wafers due to collisions.
Another object of the invention is to provide a method and apparatus as described above which increases wafer throughput and yield, while simultaneously reducing down time of the equipment resulting from wafer damage or equipment collisions.
A still further object of the invention is to provide a method and apparatus as mentioned above which includes a positive interlock feature that inhibits movement of a wafer transferring robotic arm if either a wafer is not preferably positioned on the end of the arm, or the arm, and its components are not properly aligned or registered with other components in the processing system.
Another object of the invention is to provide a method and apparatus as mentioned above which includes a minimum number of components and which can be easily retrofitted to existing wafer transfer equipment.
These, and further objects and advantages of the present invention will be made clear or will become apparent during the course of the following description of a preferred embodiment of the invention.